1. Field of the Invention
The present invention relates to an image forming apparatus which forms and develops an electrostatic latent image on an image carrier, and transfers and fixes the developed image on a sheet material to form an image, and a remaining sheet detection method for the apparatus.
2. Description of the Related Art
A conventional image forming apparatus such as a laser printer or copying machine attaches a paper cassette which stores a plurality of stacked sheets, and picks up the sheets in the paper cassette one by one and conveys the picked-up sheet into the apparatus body when printing an image. In such an apparatus, the conveyed sheet is sandwiched between a registration roller pair located on the upstream side of a transfer unit, and fed to the transfer unit by rotation of the registration roller pair. A top sensor which detects the leading edge of the sheet is arranged on the downstream side of the registration roller pair. When the top sensor detects the arrival of the sheet, the conveyance of the sheet is synchronized with an image forming operation on a photosensitive drum.
A loop sensor is arranged between the transfer unit and a fixing unit (see Japanese Patent Laid-Open No. 2000-352850). This loop sensor detects the slack (also referred to herein as a loop) of a sheet between the transfer unit and fixing unit while the leading edge of the sheet on which an image has been transferred is fed to a nip portion between a fixing roller pair but the trailing edge of the sheet has not passed through a nip portion in the transfer unit (between the transfer rollers). The rotating members (rollers) of the transfer unit and fixing unit are driven by separate motors. The rotational speed of the motor which drives the roller of the fixing unit changes in accordance with a signal detected by the loop sensor. That is, the loop sensor is turned on when the loop amount of the sheet between the transfer unit and fixing unit reaches a predetermined amount and the sheet pushes down the loop sensor. In this case, the image forming apparatus controls to decrease the loop amount by increasing the rotational speed of the roller of a fixing unit. On the other hand, the loop sensor is turned off when the loop amount decreases. In this case, the image forming apparatus controls to increase the loop amount by decreasing the rotational speed of the roller of the fixing unit. The rotational speed of the motor which drives the fixing unit changes in accordance with ON/OFF of the signal detected by the loop sensor, thereby changing the conveyance speed of the sheet conveyed by the fixing unit. Therefore, the image forming apparatus controls to keep the loop amount of the sheet almost constant between the transfer unit and fixing unit.
A delivery sensor is arranged on the downstream side of the fixing unit. The above-described top sensor and the delivery sensor function as sensors which detect the presence/absence of a sheet, and are used to detect a remaining sheet in a conveyance path in case of a jam or the like.
However, the conventional image forming apparatus with the above-described arrangement has the following problems. That is, the conventional loop sensor is a sensor dedicated to control the loop amount of a sheet when conveying the sheet. More specifically, the loop sensor is used only for control of the loop amount, and there is no means to detect the presence/absence of a sheet between the top sensor and delivery sensor when the image forming apparatus stops. Hence, when a jam occurs by a sheet remaining between the transfer unit and fixing unit, the sheet may not be detected. When the sheet remaining between the transfer unit and fixing unit is not detected, the image forming apparatus may perform a forward rotation operation to initialize the apparatus body without consideration of the jam. In such a case, the recovery process from the jam may be difficult to perform because, e.g., the remaining sheet winds around the roller of the fixing unit.
Adding another sensor for detecting a jam requires various modifications and changes such as an increase in the number of units and a change in a print engine, resulting in an increase in the cost of the product.